1. Field of the Invention
The present disclosure relates to a display structure and a method for fabricating the same, and more particularly, to the structure of a liquid crystal display device and a method for fabricating the same.
2. Description of the Prior Art
A conventional liquid crystal display (LCD) device generally includes a pair of substrates, in which the substrates are parallel and separated from each other by a distance. The space between the substrates is usually referred to as the liquid crystal cell gap or, simply, cell gap. Additionally, a liquid crystal material is disposed between the two substrates within the cell gap, in which the liquid crystal material is able to respond to an outside electronic signal, thereby altering its optical characteristics. The electronic signal is controlled by a plurality of electrodes disposed on the inner surface of the substrates. The electrode arrangement of some particular LCD devices is able to generate a group of predetermined characters or symbols whereas the electrode matrix of other LCD devices is able to produce a display image. The display image essentially includes a large number of pixel devices, in which each pixel device can be optionally turned on to generate an image with many variations.
In order to ensure proper operation of the LCD device, the cell gap has to be maintained precisely and uniformly, since imprecision within the cell gap can easily result in a defective display image (a condition usually referred to as mura). Moreover, a slight touch from a finger tip will also influence the image of the display. To respond to such pressure, the cell gap within the influenced area is slightly decreased, thereby reducing the contrast and intensity of the dark spot or any other side effect in the display image.
As shown in FIG. 1, a conventional LCD display includes a thin film transistor (TFT) substrate 61, a color filter (CF) substrate 71, and a liquid crystal material 69 disposed between the two substrates. The cell gap is usually maintained by a plurality of spacers 79 formed between the substrates 61 and 71. Disposed randomly via a dispersion technique within the cell gap, the spacers 79 are equal in height, which often result in partial uneven distribution of the spacer density. In order to maintain a uniform cell gap and the concentration of the spacers, an exceedingly higher number of spacers is often utilized. Additionally, the spacers are often disposed in both inactive and active areas of the display panel according to the conventional method. Located between two corresponding electrodes on the substrate, the active area is an area where the liquid crystal material can be selectively activated, whereas the liquid crystal material in an inactive area is unable to be selectively activated as the area lacks a pair of corresponding electrodes.
In general, the structure and function of a LCD display utilized by the conventional spacer technique results in several unwelcoming features. For instance, the spacers located within the active area will likely result in numerous disadvantages including reduced contrast or abnormal light irradiation in proximity to the edge of the spacers.
According to the European Patent No. 1,030,211 A2, an LCD utilizing a method of eliminating the step of dispersing the spacers thereby preventing the uneven distribution of spacers and difference in cell thickness is disclosed. As shown in FIG. 2, the LCD device includes a TFT substrate 30, a CF substrate 40, and a liquid crystal layer 49 sealed in between the two substrates. Additionally, protruding patterns formed on the CF substrate 40 are spacers 45, in which the spacers are approximately 4 μm in height and utilized for maintaining the uniformity of the cell gap.
In general, the cell gap of an LCD is the average distance between the two alignment films of two substrates, in which the average distance often equals to the height of the spacer after the two substrates are combined.
Nevertheless, the spacer 45 from the conventional technique or the European patent are disposed on a glass substrate 41, in which a color filter 43 is disposed in between the spacers 45 and the glass substrate 41.
Please refer to FIG. 3. FIG. 3 is a stress-strain diagram showing the relationship between a 20Φ μm columnar spacer directly disposed on a glass substrate and a columnar spacer disposed on a glass substrate having a color filter disposed in between. As shown in FIG. 3, the spacers disposed directly on the glass substrate are mostly elastomers, as shown by curve A, whereas other spacers disposed on the glass substrate having color filters in between are partially elastic, as shown by curve B, in which a permanent strain d will be generated when a loading is released from the spacers. If the loading (a pressure or stress) of the LCD device is big enough to generate the permanent strain, the total height of the spacer and the color filter disposed on the glass substrate will be altered. In other words, after a relatively large pressure is applied to the LCD panel, the uniformity of the cell gap will be disrupted, thereby resulting in effects such as mura.
In addition, numerous methods have been introduced regarding the deposition of the liquid crystal material between two substrates of an LCD. For instance, necessary components of a display panel including the thin film transistor, circuits, and color filters have to be prepared first. Next, epoxy is utilized to laminate the two substrates together, in which the substrates are separated by a distance of about 5 μm. Next, the laminated substrate is placed in a vacuum room with a container holding liquid crystal materials, in which the gap of the laminated substrate is situated in a vacuum environment. Next, the laminated substrate is moved towards the liquid crystal container and by disrupting the vacuum, the liquid crystal material is slowly absorbed by the laminated substrate via a capillary effect and the pressure outside the substrate.
In recent years, a more advanced deposition technique referred to as one drop fill has been introduced. According to this technique, a liquid crystal material is dropped into one of the substrates before two substrates are laminated. U.S. Pat. No. 5,263,888 issued to Teruhisa Ishihara et al. on Nov. 23, 1993 describes a method of manufacture a liquid crystal display panel, in which the one drop fill technique is disclosed.
One of the most influential factors regarding the one drop fill technique is the elasticity of spacers. When the number of spacers is too small or the elastic deformation is too large (such as a very small elastic coefficient), the volume of the liquid crystal is likely to overly increase, thereby resulting in gravity mura. Conversely, when the number of spacers is too large or the elastic deformation is too small (such as a very large elastic coefficient), the volume of the liquid crystal will overly decrease, thereby generating air bubbles. Ideally, spacers with much bigger height will be able to obtain a much better operation window.